[Note by Chris Long, 7 April
2005: Just
after Mike Groth published this article, he worked in nuclear medicine
at
the Brisbane General Hospital, and part of his duties involved the
maintenance
of whole-body scanners using low light level detectors, scintillation
counters
and the like. I wrote to him at that time, and the subsequent exchange
of
information began a loose experimental collaboration on optical
communication
that continued through three decades.

Having used photomultipliers and
modulated high
pressure mercury arcs in the 1970s, I was initially sceptical that
semiconductor
technology could span longer distances through the
atmosphere. Mike gave me a push into semiconductor sources and
detectors and I was amazed at their improvement since my earlier
optical
comms experiments
with VK3ZGJ (now VK3EGG) in 1976. Also in the late 1980s, really high
output
'Stanley' 2000 mCd and 4000 mCd LEDs came onto the market. In 1991,
I used Mike's semiconductor receiver circuits, fresnel lens collimators
and
the new ultra-bright 5 mm LED's to span 43 km from my old QTH in East
Hawthorn to
Peter Wolfenden VK3KAU's QTH in Sunbury. A decade later, with the
advent of 1
watt 'Luxeon' LEDs in 2001, the modulatable light flux available
increased by almost two orders of magnitude. With
these 'Luxeons', Mike and I finally spanned 167 km
in Tasmania on 19 February 2005.

Mike's original 1987 article
gave very conservative
estimates of potential operating ranges for
optical communication systems. New light sources and large-aperture
fresnel optics
have improved on these distance estimates by almost a
factor of ten.

We're putting Mike's original
1987 article on the net as a
matter of technical and historical interest, as the principles and
calculation methods are still perfectly applicable to current optical
communication systems.
In some ways, this publication marked the commencement of serious
Australian amateur work
in this field. Its comprehensive nature also makes the article a model
of its type:]

INTRODUCTION

Apart from limited military
applications, optical
telephony remained
a relatively impractical form of communication from the invention of
the photophone in 1880, to the development of
semiconductor light sources and detectors in the 1960s. While optical
fibres have become
a major component of modern telecommunications, and infra-red remote
controls are incorporated in many domestic appliances, optical
communication has been largely ignored by radio amateurs.

Construction projects for
photophones have been
published from time to time over the last 60 years, but there have been
few reviews of optical communication and its potential as
a medium for amateur voice and data communication. This article is a
mixture of history, theory and personal experience, written with the
intention of introducing optical communication to the general body of
radio amateurs and possibly stimulating further experimentation in the
oldest branch of wireless.

HISTORICAL DEVELOPMENT

Early Developments, 1878 - 1918

The invention of the selenium cell
in 1872 and the
telephone in 1876, made it possible to detect modulated light,
and Mr A.C. Brown of London is generally credited with the first
transmission of articulate speech over a light beam in 1878. Much of
the pioneer work in optical telephony was carried out by Alexander
Graham Bell and
Charles Sumner Tainter during 1879 and 1880, which was presented in a
paper1, read by Bell to the
American Association for the Advancement of Science in August 1880.

The Bell photophone (Figure 1) used
a flexible plane
mirror mounted at the end of a speaking tube, so that the sound
pressure caused the mirror to change shape, modulating the beam
intensity of the reflected light. The receiver was a selenium cell
mounted at
the focus of a parabolic reflector, and coupled to a battery and
telephone receiver. Using this apparatus, Bell transmitted
speech over a distance of 213 metres using sunlight, and shorter ranges
were covered using various lamps as a light source.

Interest in photophones appears to
have been dormant
until the turn of the century, when German and Austrian experiments
with current modulated carbon arc lamps, led to the production of a
military photophone by the Siemens-Halske company in 1917. This
unit used a current modulated carbon arc transmitter, and a selenium
cell receiver, to give a night range of about 8 km. The German Navy was
reported to have used voice modulated searchlights for ship to ship
communication up to a distance of 7 miles (11 km).

The British were also active in
photophone research
during the First World War, and the vibrating mirror modulator was
developed by
Rankine as part of a research project for The Admiralty in 19162.
Other methods of producing modulated light, including
current modulation of carbon arcs and fine filament lamps were found to
have very poor modulation characteristics.

The selenium cell was the only
photoelectric detector
available until the development of the thalofide (oxidized thallous
sulphide)
and molybdenite detectors in 1917. These had a lower noise level than
selenium and a faster response to infra-red radiation.

An experimental photophone was
developed in the U.S.A.
by the Case Research Laboratories in 1918, which used a pressure
modulated
acetylene lamp (Figure 4) in the transmitter, and a thalofide cell with
a valve amplifier in the receiver. A clear night range of 8 km
was claimed with 24" (600 mm) reflectors at each end.

1919 - 1935

Improvements were made to optical
modulators and
detectors in the
1920s, by motion picture engineers developing the optical sound tracks
on movie films. Photophones became a technical novelty for
display at industrial exhibitions and science fairs, with the
occasional construction project in the popular radio
magazines.

Military Photophones 1939- 1950

There was a renewed military
interest in optical
telephony in the 1930s, and the German Army introduced the
Zeiss Lichtsprecher infra-red photophones in 1935. The light source was
a tungsten filament lamp with an
infra-red transmitting filter, which was modulated by a vibrating
mirror (or prism in the Li80). The
receiver used a lead sulphide detector with an infra-red filter, and a
valve amplifier. They were virtually unaffected by daylight,
with a clear weather range of 3 km for the Li 50/60, to nearly 14 km
for the Li 250/130.

The Japanese Army visible light
photophone incorporated
a vibrating mirror modulator and a caesium photocell
detector, with an operating range of about 1 km in daylight and 2.5 km
at night. An Italian Army photophone
used a current modulated filament lamp as the light source, but few
details appear to have been published outside of the military reports.

Both the German and American Navies
used high pressure
vapour lamps as modulated infra-red sources for navigation,
identification and short range communication. The Germans employed
mercury arc lamps of 500 to 2000 W, while the Americans developed the
caesium
arc lamp. Some military laboratories continued the development of high
pressure arc lamps for optical communication until the 1950s.

Post War Amateur Developments

From 1945, the occasional letter
appeared in the amateur
journals describing experiments with current
modulated light globes, but with the development of transistors and
photodiodes there was a small but scattered group of amateurs
experimenting with photophones in the 1960s. Most equipment used
current modulated torch globes
and phototransistors to transmit distorted speech. but some optical
links using gas discharge tubes could transmit high fidelity speech and
music.

Following the invention of the
laser and infra-red light
emitting diodes. there was an increased amateur interest in optical
transmission between about 1966 and 1972. when several speech and video
contacts, were made over distances of 100 km or more.
Despite the rapid advances in the commercial application of optical
communication since
1970, there has been little serious interest in extending amateur radio
into the optical part of the electromagnetic
spectrum.

OPTICAL THEORY

It has been assumed that the
readers of this article have a basic understanding of optics, including
the properties of lenses
and mirrors. A simple description of some more advanced optical
concepts has been included to assist in the later discussion of
light sources, detectors, and optical systems.

Light may be loosely defined as
electromagnetic radiation having a
wavelength between 300 nm (3 x 10-7m) and 3µm (3 x 10-6m),
which corresponds to
a frequency range of 1014 to 1015 Hz. This
definition includes visible light with
a wavelength between 400 nm and 700 nm, as well as the long wavelength
ultra-violet and near infra-red
parts of the optical spectrum as shown in Figure 2. Optical
communication systems usually operate in the visible or near
infra-red.

Light is emitted and absorbed in
small discrete energy quanta called photons. The energy carried
by each photon is
determined by its frequency or wavelength according to the formula;

The spectrum of a light source
reflects the energy of
the excited electrons, and the thermal electrons in a hot body emit
broad-band radiation, whose dominant wavelength is a function of the
absolute temperature as shown in Figure 3. The 2500°K curve is
representative of the spectrum of the white light from a filament lamp
or incandescent gas mantle.

The monochromatic light from a
sodium vapour lamp, neon
globe or led, has most of its radiant energy concentrated into a
limited range of wavelengths, determined by the differences in the
atomic energy levels in the source. A monochromatic light source has
some advantages in an optical communications system, as it allows the
receiver to be tuned to the transmitter's
wavelength.

The short wavelength limit for an
optical link is set by
atmospheric absorption of ultra-violet wavelengths below 300 nm,
and the long wavelength limit is set at about 3 µm by thermal
background radiation and rising detector noise. Glass lenses and
windows are transparent to wavelengths from 350 nm to nearly
2.5 µm, while quartz will transmit infra-red to 3.5 µm.
Most
transparent plastics are suitable for infra-red operation out to a
wavelength of
2 µm (2000 nm).

OPTICAL TRANSMITTERS

Optical Intensity.

An optical transmitter generates a
beam of intensity
modulated light, either by modulating the intensity of a light source,
or by passing the light from an unmodulated source through an optical
modulator. In either case, the effectiveness of the transmitter is a
function of the transmitter's beam intensity, and the depth of
modulation.

Because light sources have a finite
size and do not
radiate equally in all directions, four parameters (see Figure 4), are
used to describe optical brightness and intensity. These are;

FLUX
(F)
The optical power (watts).

INTENSITY
(I)
The power radiated per unit solid angle in
a given direction. (watts.steradian-l).

For a point source of intensity I
radiating equally in all directions, the total flux radiated is
4.π.I watts.

Visible light photometry is based
on a white light
standard (the candela), and visual brightness comparisons between light
sources. The unit of luminous flux is the lumen, and a light source
with a luminous intensity of 1 candela, is emitting one lumen of
visible light per steradian. The candela replaces the older unit of the
candle-power, originally based on the intensity of a sperm wax candle.

A watt of green light at the
wavelength of peak response
of the human eye (555 nm), is equivalent to a luminous flux of 692
lumens. The luminous efficiency for light of other wavelengths is
reproduced in Figure 5, which may be used to estimate the radiant power
from luminous flux measurements.

Transmitter Optics.

The simplest form of optical
transmitter consists of a
modulated light source mounted at the focus of a lens or mirror as
illustrated in Figure 6. The intensity of the transmitter beam is given
by;

G .D2
Ibeam
=
__________ . Isource
ds2

Where G is a geometric correction
factor for the f/D
ratio of the optical system (Figure 7). Provided the focal length is
not too short, the output lens (or mirror) will have the same luminance
as the source, and the beam intensity will be a function of the source
luminance and the lens area.

The divergence of the transmitter
beam (θb)
is determined by the ratio of the source diameter and the focal length.
The use of a more intense source with the same luminance will increase
both the power and divergence of the transmitter beam, but the beam
intensity will remain unaltered. An optical system with a very low f/D
ratio (such as a deep parabolic reflector)
will give a very high beam power, but it can be seen from Figure 7 that
the beam intensity will be less than that produced by a lens (or
mirror) of the same
diameter and moderate focal length. This apparent contradiction arises
because the beam divergence increases at a greater rate than the total
beam power as the focal length is
reduced:

A very narrow beam can make the
transmitter difficult to
align, especially in an infra-red system where the beam is invisible.
For an optical transceiver, the transmitter beamwidth should be wider
than the receiver's field of view, so that the transmitter will be
correctly aligned when the receiver is aimed for the maximum signal.

Modulated Filament Lamps.

A tungsten filament lamp has a high
luminance in the visible and near infra-red (typically
105 W.m-2.sterad-l), but the poor
modulation of the light output (Figure 8), reduces the effective
modulated luminance to the order of 100
W.m-2.sterad-l.

Despite the low depth of
modulation and considerable
distortion, current modulated torch globes were widely used in amateur
photophones for voice communication over distances up to a kilometre on
a clear
night.

Gas Discharge Lamps.

Low pressure gas discharge lamps,
including neon bulbs
and fluorescent lamps, can be modulated to 10 KHz or more, but their
luminance is very low (typically 10 to 20
W.m-2.sterad-l ). A gas discharge has a
non-linear relationship between voltage, current and light output, but
speech and music can be reproduced with reasonable fidelity using pulse.

High pressure sodium and mercury
vapour
lamps are widely used for floodlight, factories and street lighting,
and are readily available with power ratings from 70 W to 2000 W. The
luminance (typically 6000
W.m-2.sterad-l ) is almost independent of the
wattage rating, and lamps of the 100 W size would be suitable for
amateur experimentation. The audio modulation characteristics of these
lamps is not known, but published data indicate that better than 50%
modulation
of the light output could be expected for frequencies up to 5 KHz.

The main disadvantages of high
pressure
lamps are the relatively high cost, limited life (500 - 2000 hours),
and
the long warm-up time. Sodium and mercury vapour lamps require at least
10 minutes operation to evaporate the metal in the lamp and produce
their full light output. An optical
transceiver with a high pressure vapour lamp, would have to run its
transmitter continuously, with a shutter to cut the beam off during
reception.

Light Emitting Diodes.

Light emitting diodes are junction
diodes made from compounds of
gallium, aluminium, arsenic and phosphorus. which emit nearly
monochromatic light when forward biased. The emission wavelength
depends on the chemical composition of the diode crystal. and ranges
from 930 nm in the near infra-red for gallium arsenide
(GaAs), to blue light at 500 nm for aluminium phosphide diodes.

The light emitting diode is the
most convenient light source currently available for amateur optical
communication.
The output is proportional to the forward current and may be modulated
to frequencies exceeding 1 MHz. The
optical properties of several common light emitting diodes are
summarised in Table
1:

It can be seen that the efficiency
and
power output of a LED decreases with the emission wavelength, and an
infra-red emitting diode has much greater output flux than a green LED
for the same drive current. A high intensity red LED is a suitable
modulated light source for demonstrations and experiments, as the
visible radiation
simplifies the optical adjustments.

High powered GaAs and GaAlAs
infra-red
emitting diodes are available with peak output powers of several watts,
but the luminance of the source is probably not significantly higher
than
for smaller diodes. The efficiency and power output of an LED is
temperature dependent (Figure 9), and some form of heat sinking is
necessary if operating a diode near its maximum current.

Most light small emitting diodes
are
supplied in a transparent plastic package with a domed top, which acts
as a lens and increases the intensity of the light along the diode
axis. The lens does not increase the source luminance, but generates a
bright halo as illustrated in Figure
10:

The effective luminance may be
estimated by assuming the source diameter is equal to the diameter of
the diode.

Mechanical Modulators.

A variety of mechanical devices
have been
devised over the past 108 years to impress voice modulation on a beam
of light. As it impossible to cover these in detail, this review has
been restricted to the basic principles of some of the more successful
designs.

The intensity of a light beam may
be
modulated by altering the optical flux in the beam with a variable
transmission device, or by changing the divergence of the beam. The
latter approach was adopted by Bell in his 1880 photophone (Figure 1),
which used
a flexible mirror to vary the divergence of the reflected beam in
sympathy with the sound pressure.

A modern version of the Bell
modulator may be constructed by mounting a sheet of
aluminised plastic or a thin glass mirror in front of a loudspeaker as
shown in Figure 11. There should be a
good seal between the loudspeaker rim and the mirror to achieve a tight
acoustic coupling.

A simple modulator for use with a
small
filament lamp is drawn in Figure 12, where the flexible mirror and the
lens form an optical system of variable focal length. The optical path
from the lamp to the lens should be slightly shorter than the focal
length, so that
the filament will be in focus at the maximum concave curvature of the
mirror. This modulator is most effective with a torch globe having a
short narrow filament.

The flexible mirror is not a linear
modulator, and the distortion rises rapidly with increasing modulation
depth. Up to
30% modulation is possible with a very flexible mirror, but a
transmitter using a glass mirror is unlikely to
achieve more than about 5% modulation of the beam intensity.

The vibrating grid modulator is
constructed from a pair of identical grids, each having equal
transparent and opaque strips.
One of the grids is fixed, and the other is attached to the voice coil
of a loudspeaker driver as shown in Figure 13. The two grids have a
static displacement of half a strip width, and driving the voice coil
with an audio signal will modulate the transmitted light power about
its quiescent value of
one quarter of the incident optical flux.

The performance of the system will
depend
on the fineness and accuracy of the grids, as well as the mass and
frequency response of the moving grid. The grids with strips about 1 mm
wide could be a pair of photographic transparencies, or etched from a
thin sheet of metal. The vibrating grid concept was independently
suggested by Alexander Graham Bell in 1880, and by Sir William Bragg in
1915, but it was impractical with the acoustic drive systems available
at the time.

The problems associated with the
moving
grids were overcome by Rankine in 1915, by using fixed grids and an
optical lever
as illustrated in Figure 14. The grids were located at the radius of
curvature of the concave mirror, which formed an image of the first
grid in the plane of the second. A small rotation of the mirror
will move this image over the second grid, and modulate the luminance
of the image formed by the second lens. The light from this image is
collimated by the output lens to produce the main transmitter beam.

The rotation of the mirror may be
produced by a high speed galvanometer, or a loudspeaker voice coil via
a lever and fulcrum.
Despite its greater complexity, the oscillating mirror modulator was
the most successful mechanical design, and was used by the Japanese
and Germans in their military photophones during the 1930's. Several
other mechanical modulators have been developed using
internally reflecting prisms or interferometers with movable plates.
They have not been included in this review as they are
precision devices which would not be suitable for amateur construction.

Electrical and Magnetic
modulators.

The Kerr cell is a glass cell
fitted with
parallel electrodes and filled with
nitrobenzine, which becomes doubly refracting in an electric field. The
cell is mounted between a pair of crossed
polarizers (Figure 15}, whose planes of polarization are at 45° to
the
electric axis of the Kerr cell. In the absence of an electric
field no light is transmitted by the second polarizer. When a voltage
is applied to the electrodes, the Kerr cell becomes doubly
refracting, and the light emerging from the cell is elliptically
polarized. As this now has a polarization component aligned with
the second polarizer, some will be transmitted.

The optical path difference between
the
two polarization components in the cell is proportional to the square
of the applied
voltage, with a response time of less than 1 ns. Very strong electric
fields are required to open the shutter, and a Kerr cell is
often operated with an RF drive, when the light will be chopped at
twice the excitation frequency.

Caution must be exercised when
experimenting with Kerr cells, as very high voltages are involved, and
nitrobenzine is very poisonous. It is also
a powerful solvent and will attack most plastics, and a fatal dose can
be absorbed through the skin.

A
magneto-optic modulator (Figure 16), utilizes the Faraday rotation of a
beam of polarized light shining along
a magnetic field. Most transparent materials exhibit
a very small Faraday rotation. and the effect is strongest in
ferro-magnetic materials.
An experimental voice modulator was developed in the 1960's using
a thin section of yttrium-iron-garnet. which is transparent to near
infra-red and exhibits
a large faraday rotation.

Lasers.

A
laser is a monochromatic light source in which the electron transitions
have been synchronized by optical feedback, so that the photons are in
phase with each other, and the light is coherent. Coherent light has
the properties of
a continuous wave, with a very narrow spectral bandwidth.

Lasers are best known for their
high
optical power output. Gas lasers producing over a kilowatt of optical
flux are in regular use in industry for cutting cloth, wood and metals.
The argon laser is widely employed for surgical operations, and solid
state lasers with peak output powers of
a terawatt (1012W) or more, probe the atmosphere and measure
distances to
satellites.

The most common laser for optical
communications is the semi-conductor or diode laser, which is a
modified infra-red emitting
diode that generates coherent radiation. The luminance is much higher
than a normal infra-red emitting diode, with a very narrow
spectral spread. The infra-red is emitted with a divergence of about
10°, and can be current modulated to several MHz.[Chris
Long notes, 8 April 2005: Laser diodes
were very expensive
when Mike wrote this article, but they are now commonly available from
around
US$15+, in a range of wavelengths from 635 nm (red) extending upwards
into the
infra-red. At longer wavelengths they can operate with considerable
output
powers, but in the visible spectrum where they're used as bar code
readers and
lecture pointers they mostly operate at about 1 to 15 mW. A few very
expensive visible
laser diodes attain 750 mW. They are not inherently collimated,
radiating
in an elliptical cone pattern, at an angle angle of about 30 degrees
parallel to
the junction, and around 8 degrees perpendicular to the junction. They
are
therefore usually supplied with a collimating lens somewhat less than 1
cm in
diameter. For atmospheric optical communication this beam diameter
would have to
be spread optically in the manner of Figure 17 for eye safety and to
reduce
far-field divergence. Their beam cross-section is not very homogenous
in
brightness and they are only moderately coherent. Gas lasers still have
the
advantage of being inherently collimated and highly coherent in output
(ie a
high proportion of their radiation has a narrowly monochromatic output
with the light waves
in phase, plane
parallel and minimally diverging - but they are much more expensive
than diode
lasers, they require an EHT supply and are difficult to modulate
internally).]

The other common laser to which
amateurs are likely to have reasonable access is the helium-neon gas
laser, which emits up to
20 mW of red light with a wavelength of 632.8 nm. The light is emitted
in a thin parallel beam, and the
He-Ne laser is widely used in teaching, science, engineering, and
surveying. The gas discharge
may be powered by a DC current or an RF signal, and a 10 metre AM
transmitter can. be used as a exciter for
photophone experiments.

The parallel beam of light emitted
by a laser will start to diverge after a short distance as a result of
diffraction, but this
can be reduced by expanding the beam through an astronomical telescope
as depicted in Figure
17:

The diffraction spreading for a
l00 mm diameter beam of coherent red light is about 15 microradians,
but an expanded laser beam is observed to diverge at nearly 200
microradians (200 mm/km or 1 ft/mile), probably as a result of
atmospheric turbulence and imperfections in the telescope.

OPTICAL RECEIVERS

An optical transmitter generates a
beam
of intensity modulated light. which is received by a photodetector and
converted directly
to an audio frequency electric current. This is similar to the early
days of amateur radio. when incoherent signals from spark
transmitters were received by crystal sets. Experimental coherent
fibre-optic receivers have been demonstrated in several research
laboratories. but a coherent optical communication system for
atmospheric transmission is not likely to be available for some
time.

Detector Theory.

A photodetector is a quantum device
which uses the photon energy of the light to excite electrons and
generate a current
proportional to the energetic photon flux. All photon detectors have a
cutoff wavelength λc
which corresponds
to the minimum photon energy required to excite an electron in the
detector. In an ideal
detector. each incident photon with a wavelength less than λc will
liberate an electron, but the quantum efficiency of a real detector
ranges from 0.03 to 0.5 electrons/photon.

The sensitivity of a photon
detector is the detector current generated per watt of incident optical
flux. It is inherently
wavelength dependent (Figure 18), with the maximum sensitivity at a
wavelength slightly shorter than λc.
Radiation with a
wavelength longer than λc
will not be detected. The short wavelength limit is
usually determined by absorption in the detector window.

A detector will generate white
noise from
electrical leakage, thermal excitation, and background light. The dark
current is proportional to the square root of the detector area. and
increases rapidly with the temperature and cutoff wavelength. The
thermal noise contribution from a detector with a cutoff wavelength in
the visible part of the optical spectrum will generally be less than
the amplifier noise. Detectors sensitive to far infra-red radiation
have a very high thermal noise level at room temperatures, and are not
particularly suitable for optical communications.

Unmodulated light falling on a
detector
will generate white noise from statistical fluctuations in the photon
flux. The light noise is proportional to the square root of the
detector current,
and is a function of the total light flux. Background light may be the
main noise contribution in an atmospheric optical link operating during
the day or on a moonlit night.

Receiver Optics.

In a typical amateur photophone
receiver,
the light from the transmitter is concentrated on the sensitive area of
the detector by
a lens as illustrated in Figure 20, although mirrors become more
convenient if a large collector is required. The lens (or mirror}
should have a focal length longer than its diameter for efficient light
collection. Magnifying glasses or magnifying sheets make suitable
receiving lenses up to a diameter of 250 mm, for visible or near
infra-red signals.

The lens (or mirror) will form an
image
of the transmitter output aperture at the focal plane, which for a lens
of reasonable
focal length. will have a diameter of less than 1 mm. As this is
smaller than the sensitive area of a practical detector, all the
transmitter light falling on the receiving lens will fall within the
active area of the detector. The detector current will therefore be
proportional to the area of the lens or mirror, and independent of the
focal length or the detector area.

A receiver will detect light
arriving
within a conical field of view, whose angular diameter is defined by
the focal length and
detector diameter. This field of view may include unmodulated light
from scattered sunlight or moonlight, as well as modulated light
from street lighting and other sources. The unmodulated light will
generate white noise in the detector, while street lights and house
lights will produce a strong 100 Hz interference.

As the noise and interference
produced by
the background light will increase with the receiver beamwidth, the
receiver's field of
view should be reasonably narrow. However, a very narrow field of view
will make the receiver difficult to align, and may require
some form of optical tracking system to compensate for changes in
atmospheric refraction.

A detector about 2 mm in diameter
will
give a beamwidth between 3 and 10 milliradians (0.2°- 0.6°)
with
typical receiver lenses,
which appears to be a reasonable compromise between interference
suppression and ease of aiming. Larger detectors should have their
effective diameter reduced with a focal plane aperture plate.

Photocells.

A photocell (Figure 21) is a vacuum
diode
whose cathode is coated with a material that emits electrons when
exposed to light. The spectral response is determined by the cathode
coating, which may be a mixture to produce a more constant sensitivity
across the visible spectrum. Most photocathodes are relatively
insensitive to red and infra-red light, but a photocell with a caesium
cathode can detect infra-red wavelengths out to nearly
1300 nm.

Photocells are large detectors,
with cathode areas from 1 cm2
to 10 cm2,
but they have a very low thermal noise, wide dynamic range, and fast
transient response. They have been successfully employed as detectors
in visible light photophones in the past, but have been largely
superseded by the silicon photodiode.

Photomultipliers.

A photomultiplier is a vacuum
photocell
fitted with a series of dynodes (Figure 22), which multiply the
photocurrent by secondary electron emission. A typical photomultiplier
has a sensitivity of the order of
105 amps/watt, and can detect a modulated light flux of 10-13W.

The photomultiplier is best suited
for
detecting faint light signals in a dark environment, and will saturate
with a relatively low level of background light. They are very
expensive ($50+) and relatively fragile devices, which can be damaged
if exposed to
a bright light with the HT applied. They are mainly used for amateur
optical DX experiments, and are not recommended for inexperienced
amateur experimenters.

Photodiodes.

A photodiode uses the photon energy
to
produce charge carriers in the depletion region of a semiconductor
junction, and generate a
current. This phenomenon is observed in several semiconductors, but the
highest quantum efficiency and lowest leakage are obtained with
a p-i-n junction, which has a wide depletion layer. A photodiode acts
as a current generator, but is often operated with a reverse
bias (Figure 23), to improve the transient response.

A silicon photodiode will detect
ultra-violet, visible and near infra-red radiation out to a wavelength
of 1100 nm. The peak
response at 950 nm is near the emission wavelength for infra-red
diodes, and many of the small photodiodes sold by electronic
component suppliers have an integral infra-red filter. Photodiodes are
well suited for optical communications, being small, cheap and
rugged, with a high quantum efficiency, and a relatively low thermal
noise level.

Measurements made by the author
indicate that a BPW50 silicon photodiode connected to a low-noise audio
amplifier can detect a
tone modulated signal of 2 x 10-11 W at a wavelength of 900
nm. An AM speech signal of
10-10 W is quite readable, while an FM subcarrier system
requires a signal flux approaching 2 x
10-10 W.

A germanium photodiode has a cutoff
wavelength of 1800 nm, with its peak response at 1550 nm, and is a good
spectral match for
detecting the light from an incandescent lamp. The noise level is
higher than a silicon photodiode, and an OAP12 germanium photodiode
requires a flux of 3 x 10-10 W to produce a readable speech
signal.

A light emitting diode may be used
as both the light source and detector in a short range photophone, as
shown in Figure 24. The
cutoff wavelength of a LED operating as a photodiode is about the same
as the emission wavelength, and the quantum efficiency is
rather low when detecting radiation from another LED of the same type:

Photodiodes sensitive to far
infra-red wavelengths have been developed using new semiconductor
compounds with a very narrow
energy band gap. These include indium arsenide (InAs), indium
antimonide (InSb), platinum silicide (PtSi), and mercury cadmium
telluride (HgCdTe), which is sensitive to radiation out to 15µm.
Many of these detectors have been developed for military
applications, and a lot of the technical data is classified.

Far infra-red wavelengths are of
limited use for optical communications due to the high thermal
background radiation between
3 µm and 50 µm. Detectors operating in this wavelength
range have to be operated at about
80° K, which requires a liquid nitrogen cooling system.

Phototransistors.

Light falling of the base region of
a
transistor will generate charge carriers, which are multiplied by the
transistor action.
Silicon photodiodes with a cutoff wavelength of 1100 nm are readily
available from electronics retailers, and are widely used in optical
isolators and position sensors. Germanium phototransistors may be
obtained by removing the opaque black paint from an older glass
encapsulated germanium transistor such as an OC70, OC71, or OC75.

A phototransistor is often operated
with
an open circuit base for maximum sensitivity, but this produces a high
noise level, as the leakage current and background light photocurrent
are amplified together with the signal. The dynamic range is limited,
and the transistor will saturate at moderate levels of background
light.
By operating a phototransistor in a bootstrapped amplifier as shown in
Figure 25, the quiescent current is stabilized by the DC feedback,
while the base impedance is very high at audio frequencies. This
circuit is relatively insensitive to
background light, but can detect a tone modulated optical flux of 200
pW (2 x 10-10W), and a speech signal of about 1 nW
(10-9W).

Germanium phototransistors (eg
OCP71)
were widely used in amateur photophones in the 1960's to detect light
from modulated filament lamps, but were largely rendered
obsolete by the development of silicon transistors. A germanium
phototransistor has a high
leakage current and noise level, but when operated in a circuit similar
to Figure 25, it should be possible to detect a speech
signal of less than 10 nW.

Photoresistors.

Several semiconducting materials
exhibit
a reduction in bulk resistivity when exposed to light. Since this is
mainly a surface effect,
a typical photoresistor is manufactured from a thin layer of
photoresistive material mounted on an insulating substrate between a
pair of conducting fingers. The resistance changes are relatively slow,
and there may be some treble cut when detecting a speech modulated
signal.

Photoresistors are the oldest form
of
photoelectric detector, and were used in all photophones until the
development of the
photocell in the 1920's. The selenium cell was the primary detector
until 1917, when it was superseded by other materials, including
thallous oxy-sulphide (Thalofide), molybdenite, lead sulphide, and
cadmium sulphide.

Photoresistors are the noisiest
class of
optical detectors, and inferior to photodiodes for visible or short
wavelength infra-red.
The lead sulphide (PbS) cell with a cutoff wavelength of 3.4 µm
is
useful for detecting radiation at the long wavelength end of the
near infra-red. The noise level is very high at room temperatures, and
it operates best at -30°C, when a speech signal of about 10 nW
(10-8W), can be detected. Dry ice, which sublimes at a
temperature of -49°C is a suitable cooling medium.

The cadmium sulphide photoresistors
sold as light dependent resistors
(LDRs), are sensitive to visible light, with a relatively slow
transient response. While they can detect a speech modulated
optical signal with reasonable fidelity, they are much noisier than
photodiodes
and most phototransistors, and are not particularly suitable as
photophone detectors.

Photoresistors using doped
germanium are used for detecting very long wavelength infra-red
radiation. The cutoff wavelength
depends on the doping element, and varies from 25 µm for copper,
to nearly 100
µm for gold doped germanium. These detectors are usually operated
at about 4°K with liquid helium cooling.

OPTICAL LINK PERFORMANCE

Atmospheric attenuation.

The optical power in a beam of
light
transmitted through the atmosphere will decrease exponentially with
distance as a result of scattering and absorption. Atmospheric
attenuation is often the dominant factor in determining the range and
reliability of an atmospheric optical link over distances of a
kilometre or more.

Provided the distance is large
compared
with the diameter of the transmitter lens (or mirror), the illumination
(E) produced by the beam at a distance R is given by;

The atmospheric attenuation
coefficient
is the sum of three main components, namely rayleigh scattering from
fine aerosols, absorption by atmospheric gases, and scattering from
large suspended particles such as fog, dust and thick smoke.

Rayleigh scattering describes the
scattering of energy by particles smaller than the wavelength, such as
air molecules and
fine aerosols. The scattering decreases with the fourth power of the
wavelength, and is responsible for the blue colour of the sky
and the blue haze observed over mountains. Red and infra-red light will
penetrate haze better than blue light, but the
transmission losses due to rayleigh scattering are relatively low.

Ultra-violet radiation is absorbed
in
air, and unsuitable for optical
communications except over very short distances. Quartz windows and
lenses are
required, as glass is opaque to wavelengths shorter than 350 nm. An
ultra-violet optical link would present a significant visual hazard to
anyone looking down the transmitter
beam without eye protection.

Atmospheric water vapour produces
strong
absorption bands in the near
infra-red, as can be seen in Figure 26, which shows the transmission
factor over a 1 km path on a fine autumn or spring day. The infra-red
absorption would be lower on a clear frosty night, but more than
doubled for a humid summer's day.

[Note by Chris Long, 8 April
2005: It
would be easy to misinterpret the graph above, as the depth of its
absorption
dips are partly a function of the averaging produced by the graph's
poor
wavelength resolution. If one narrows the bandwidth of the measuring
light
source to obtain a higher resolution graph of atmospheric transmission,
it would
have sharper and more numerous dips in its transmission curve,
like the graph below. Figure 26A is based on a computer simulator
program
developed by the United States Air Force in the early 1980s, into which
various
figures of distance, humidity etc can be fed to provide a prediction of
transmission at various wavelengths for a wide range of conditions. For
the
graph below, the CSIRO Atmospheric Physics Division in Aspendale (Vic.)
was kind
enough to feed various atmospheric 'scenarios' into their computer for
us, to
predict a fair range of possible results under differing weather
conditions. The
graph below would be typical on a clear, dry day, and has a wavelength
resolution similar
to that of LED sources:

[Even the graph above,
sufficiently accurate
for non-coherent sources of relatively narrow bandwidth like LED's
(around 30 nm
wide), is nowhere near the fine wavelength resolution required to
predict the
atmospheric transmission of something like a gas or ruby laser. The
graph below
was produced by 'tuning' a ruby laser, which can be done by varying the
temperature of its emitting ruby rod - see the graduations in degrees
Kelvin
below. A similar high-resolution atmospheric transmission model for the
whole of
the visible and infra-red spectrum is now believed to be available in
the USAF's
"HITRAN" freeware computer program. The incredibly numerous and sharp
atmospheric absorption wavelengths are
a troublesome factor in predicting the 'transparency' of the atmosphere
for a
given laser's output. To produce a reproducible atmospheric
transmission figure,
frequency control via fine temperature control of the laser rod would
be
essential. These problems do not occur so much with non-coherent
sources.]

[We now revert to Mike Groth's
original article - C.L.]:

Atmospheric absorption from rain,
mist,
fog, smoke or dust, is the main limitation on the reliable operating
range of an optical link, and there is no significant difference in the
transmission of infra-red and visible light in fog or rain. It is not
possible to
predict the signal loss due to adverse weather conditions with any
precision, but a rough estimate of the attenuation coefficient may
be made from the daylight visual range with the aid of Table 2.

Background light.

Background light falling on the
detector
will generate white noise, which is often the main noise contribution
in an optical receiver operating during the day. The detector current
from the background light will be a function of the brightness of the
background at the operating wavelength, the receiver beamwidth, and the
spectral response of the detector and optical filters.

The reflectance and colour of the
background will depend on the transmitter environment (trees, sky,
buildings, etc.), and the ambient illumination will vary with the
weather and the time of day. However it is possible to estimate
background light levels for special cases, so that the daylight
performance of different systems can be compared.

The albedo or average reflectance
of the earth is about 0.3, and the solar illumination at the surface is
1100
W/m2. It has been assumed that the background at noon on a
fine day has a brightness
of 330 W/m2, which is equivalent to a luminance of 50 W m-2
ster-1. The corresponding spectral radiance R.λ, is plotted in Figure
27:

[Note by Chris Long, 7 April
2005: I have added Figure
27A, reproduced immediately above, to give a more detailed view of
expected
daylight background radiation. During most of the daylight hours, solar
radiation at sea level peaks around the wavelength of green light - 500
nm -
which, perhaps not surprisingly, roughly matches the peak response
wavelength of
the human eye. The solar irradiance drops off rapidly at the red end of
the
spectrum and it falls even further in the near infra-red. Solar
background radiation drops to 30%
of the green wavelength peak at 1000 nm. The light scattered by
the sky falls even more rapidly with increasing wavelength, and the sky
appears
almost black in infra-red photographs. The amount of undesired daylight
background radiation detected by an optical receiver will vary
according to the
background 'seen' around the transmitter site by the receiver's optical
system.
Green foliage reflects infra-red light strongly while sky light does
not. A
simple red Wratten filter (sold by photographic dealers for black and
white
special effects as the 'red 25' or 'red 25A' filter) placed over the Si
PIN
diode will therefore help to reduce this daylight background, and a
narrow-band
filter centred on the transmission wavelength may be even more
effective.
However, communication systems should avoid the wavelengths shaded on
the graph above,
which represent atmospheric absorption bands mostly attributable to the
spectral
lines or resonance bands of H2O, CO2, O2
and 03 (ozone). These obviously
reduce the solar irradiation received at ground level on those
wavelengths,
which might be desirable, but these bands would also absorb a modulated
light beam travelling horizontally
near sea level to a far greater degree.

At night the light loss caused
by an optical filter will
often outweigh
its contribution to signal recovery, and optical filtration then ceases
to be
useful. Back to Mike's
narrative...]

The background luminance on a
heavily overcast day, may be less than 10 W
m-2 ster-1,
and at sunrise and sunset, the solar illumination is about 1% of its
noon value. The full moon is about
a million times less bright than the sun, and the background radiance
in these cases may be estimated by dividing the values read from Figure
27 by the appropriate factor.

It can be shown that the flux
reaching the detector of an optical receiver operating with a
background radiance
R.λ, is given by;

It can be
seen that an optical receiver for operation in the presence of
background light should have a narrow field of view, and an optical
filter centred about the operating wavelength. A very narrow band
interference filter
(less than 3 nm bandwidth) can be used with a gas or injection laser,
but a wider bandwidth
(around 30 nm bandwidth) is required to transmit the radiation from a
light emitting diode. A simple red or infra-red filter will give a
significant improvement in the signal to noise ratio when detecting
radiation from an incandescent light source.

Typical
values for the background light flux and noise level for several common
detectors and filters, are given in Table 3.
This table assumes a lens diameter of 100 mm and a focal length of 250
mm. For lenses or mirrors having a significantly different f/D
ratio,
the NEP from the table should be multiplied by 2.5 D/f.

For other weather or lighting
conditions, the detector flux and noise power can be estimated as
follows;

Overcast
day:
Divide
Wbg
by
10.
N.E.P by 3.

Sunrise or
sunset:
Divide
Wbg by
100.
N.E.P by 10.

Full moon at
zenith: Divide
Wbg by 1,000,000. N.E.P
by 1000.

Moonrise or
moonset: Divide Wbg by
108.
N.E.P by 10000.

Operating Range.

The
operating range of an optical link is dependent on the weather and time
of day, and any quoted range must be qualified with the appropriate
operating conditions. The vacuum range is the theoretical
communications range in the absence of atmospheric absorption, and is a
convenient parameter for expressing the optical performance of a given
transmitter and receiver. The operating ranges for various conditions
can be estimated from the vacuum range and the atmospheric attenuation
coefficient, as illustrated by the following example.

A simple photophone transmitter
consists of a current modulated Tandy XC880
GaAlAs infra-red diode mounted at the focus of a 100 mm magnifying
glass (f = 250 mm). The transmitter beam intensity
may be calculated as follows.

The source intensity is calculated
by assuming the radiation is emitted into a 24° cone, when:

The transmitter intensity and
beamwidth will be;

If the detector is a BPW50
infra-red photodiode. then a signal flux of
10-10 W will be required for speech reception. For a 100 mm
diameter lens. the minimum transmitter illumination will
be;

The operating range (OR) can be
obtained from the equation:

Log10(VR) = Log10(OR) + OR x
[Transmission loss
(dB/km)] /20

This
equation does not have a simple analytic solution for the operating
range. but can be solved by successive approximations.
From table 4. the clear air transmission loss at a wavelength of 880 nm
is 0.8 db/km. which gives an operating range of 7.4 km on a
clear night.

In the middle of
a fine sunny day, the background light noise for a BPW50 infra-red
photodiode at the focus of
a 100 mm lens (f = 250 mm), would be about
2 x 10-10 W (Table 3). In this case, a signal flux of the
order of 10-9
W will be required for speech reception. Repeating the previous
calculations with
Wmin
= 10-9 W, will give a vacuum range of 4.3 km, and
a clear air daylight range of 3.3 km.

The background light noise at
sunrise and sunset will be about the same level as the detector and
amplifier dark noise. Assuming
a total noise level of 5 x 10-10 W, and
a minimum useful signal of 2 x 10-10 W for speech
communication, the clear weather twilight
range would be 5.9 km. The background light from
a full moon would produce a detector noise of less than 10-12
W, which is much
smaller than the typical receiver dark noise, and moonlight will not
significantly effect the operation of this optical link.

The effect of water vapour may be
illustrated by repeating these calculations for an optical link using
a CQY89 GaAs LED as the light source. The intensity of the CQY89 (7.5
mW/sterad @ 50
mA), is similar to the XC880, giving
a vacuum range of 13.7 km. The emission wavelength of 930 nm, is on the
edge of
a water vapour absorption band, and the predicted operating range is
reduced to between 4 km on a humid summer evening, and
6 km on a frosty night. The predicted clear daylight range is 2.2 km to
2.9 km, depending on
the humidity.

The
estimated clear weather operating ranges for various optical
communications systems are listed in Table 5, and it can be seen that
quite simple equipment can transmit speech or data over distances of 2
or 3 km in clear weather. Over long optical paths, the received signal
strength is primarily determined by the atmospheric attenuation, and
very intense transmitter beams are required for long distance optical
communication.

Reliability.

The
reliability of an optical link depends on the path length, as well as
the frequency and severity of adverse atmospheric conditions. Signal
losses of 200 dB/km at a wavelength of 900 nm have been
observed by the author during thick fog, with fluctuations of 30 dB/km
over periods of a few seconds. Under these conditions, an optical link
using infra-red LEDs and
photodiodes would have an operating range of about 180 metres. A
modulated gas laser beam would be readable at 500 metres, and a
transmitter using a 100 W quartz-iodide lamp would have a range
approaching 280 metres.

Therefore
an optical link using simple components can provide reliable
communication over distances of 100 to 150 metres in all weather
conditions. Signal dropouts would be experienced during heavy fog at
200 metres, while a light fog or heavy rain would disrupt
communications over a 1 km optical link. Depending on the equipment
used, amateur photophone contacts of 5 to 10 km could be expected on
clear nights, with possible DX contacts of 50 km or more under suitable
atmospheric conditions.

OPTICS, RADIO AND WIRELESS.

The use of
light to transmit information was a form of wireless communication
under the broad definitions employed in the Wireless Telegraphy Act,
but the 1983 Radio Communications Act defines a
radio transmission as:-

(a) any transmission or emission of
electromagnetic energy of frequencies less than 3
terahertz without continuous artificial guide; or

(b) any highly coherent
transmission or emission of electromagnetic energy of frequencies not
less than
3 terahertz and not exceeding 1000 terahertz, without continuous
artificial guide.

This
definition excludes incoherent optical signalling systems, such as
amateur photophones or infra-red remote control systems, but a
commercial laser powered optical link is a radio system, and requires a
licence.

At present
there are no Australian frequency allocations above 300 GHz, and it
would assist in the orderly development of the sub-millimetre spectrum,
if the W.I.A approached the Department of Communications with a
proposal for amateur allocations above
300 MHz. This application could include reasonable use of coherent
radiation from 100 THz to 1000 THz
(3 µm to 300 nm), for amateur communications experiments.

CONCLUSIONS

Optical
communication is a practical method for transmitting information over
short
distances, and is used commercially for computer links between city
buildings, or across
roads, where it is not practical or economic to use a wire or radio
circuit. Optical links would be well suited for linking amateur
computers, especially between apartment blocks where RF links can cause
interference with adjacent entertainment and security systems. An
optical packet message system would be tolerant of the occasional
signal dropout caused by
rain, fog, or birds flying through the beam, and it is not impossible
to visualise the future establishment of an amateur optical packet
network in the high-rise residential systems of the capital cities.

Optical DX
can provide a challenge to the radio amateur or
experimenter, who likes to do things the hard way. Optical voice
and data transmissions of 100 km or more have been achieved in the
past, and optical moonbounce is technically possible. In many
ways,
optical communication has come of age after a century of retarded
development, and is both the oldest and one of the newest branches
of amateur radio.

ADDITIONAL READING

This review
is a distillation of information gathered from many sources by the
author over a period of 18 years, and it would be impossible to give a
comprehensive listing of references. Much of the theory can be found in
standard physics textbooks, but the following references make
interesting background reading, and
provide a suitable starting point for a detailed literature search
if desired.

1. A.G. Bell: "On the Production
and Reproduction of Speech by Light" American Journal of Science,
Vol 20, No 118, Pages 305- 324, (October 1880)